/Engine/Source/Runtime/Core/Public/Misc/ConfigCacheIni.h
Saves only the sections in thisFConfigFileinto the given file. All other sections in the file are left alone. The sections in this file are completely replaced. If IniRootName is specified, the current section settings are diffed against the file in the hierarchy up to right before this file (so, if you are saving DefaultEngine.ini, and IniRootName is "Engine", then Base.ini and BaseEngine.ini will be loaded, and only differences against that will be saved into DefaultEngine.ini)
void UpdateSections ( const TCHAR * DiskFilename, const TCHAR * IniRootName, const TCHAR * OverridePlatform )